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The speed of light in the vacuum of free space is an important physical constant usually denoted by the letter c. It is the speed of all electromagnetic radiation, including visible light, in free space. It is the speed of anything having zero rest mass. In SI units, the speed of light in a vacuum is exactly 299,792,458 metres per second (1,079,252,849 km/h). Experimental evidence has shown that the speed of light is independent of the motion of the source. It has also been confirmed experimentally that the two-way speed of light (for example from a source, to a mirror, and back again) is constant. Used to measure a light year. (Wikipedia)